Copper powder

ABSTRACT

A copper powder that is excellent in weatherability and adapted for use in conductive paste is provided that contains 10-20,000 ppm, preferably 100-2,000 ppm, of Sn. The copper powder is particularly preferably one having an average particle diameter DM of 0.1-2 μm and, further, one wherein the particle diameter of at least 80% of all particles is in the range of 0.5 D M -1.5 D M . This copper powder can be produced, for example, by precipitating Cu metal by reduction of Cu ions in the presence of Sn ions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a fine copper powder suitable for use asfiller in a conductive paste or the like, particularly to such a copperpowder having improved weatherability.

2. Background Art

Conductive pastes are widely used for forming electronic circuits andthe external electrodes of ceramic capacitors. Typical conductivefillers used in conductive pastes include copper, nickel, silver and thelike. Among these, copper is used extensively nowadays because it isinexpensive, low in resistance and excellent in anti-migration property.A conductive filler comprising a mixture of copper powders of variousparticle diameters is usually used in a conductive paste for theexternal electrodes of a ceramic capacitor. However, in order to form adense film for improving electrode reliability, the copper powder priorto mixing needs to be one of high fineness, e.g., of a particle diameterof not greater than 0.5 μm, and of uniform particle size.

Methods available for copper powder production include, for example, theatomization process, mechanical crushing process, electrolyticdeposition process, vapor deposition process and wet reduction process.The wet reduction process is the main one used today because it issuperior in the point of enabling efficient production of a copperpowder that is composed of fine spherical particles having a narrowparticle size distribution and, as such, is suitable for use in aconductive paste. For example, the prior art includes processes forobtaining fine copper powder by using hydrazine to reduce copper oxide,as taught by JP 10-330801A, JP 1-290706A and JP 5-57324B.

SUMMARY OF THE INVENTION

The prior art processes enable production of a fine copper powder havingrelatively uniform particle size. However, the use of electronicequipment utilizing conductive paste in an increasingly broad range ofapplications in recent years has led to a need for conductive pasteswith aging deterioration resistance property capable of ensuringlong-term, stable operation of electronic equipment in variousenvironments. A need has also arisen for the copper powder used as aconductive filler to exhibit stability of surface condition andresistance against aging deterioration during the storage period betweenproduction and use in a paste. Basic properties strongly desired of acopper powder for use in conductive paste have therefore come to includeresistance to oxidation during storage at room temperature, i.e.,excellent weatherability.

Nevertheless, the response to the need for improvement of theweatherability of copper powder for use in conductive paste has so farbeen inadequate. The present invention is aimed at developing andproviding a fine copper powder for conductive pastes that is uniform inparticle size and high in weatherability.

Through various studies, the inventors learned that a copper powdercontaining a suitable amount of tin (Sn) exhibits a marked improvementin weatherability. Specifically, the present invention provides a copperpowder containing 10-20,000 ppm, preferably 100-2,000 ppm, of Sn. Thecopper powder is particularly preferably one having an average particlediameter DM of 0.1-2 μm and, further, one wherein the particle diameterof at least 80% of all particles is in the range of 0.5 DM-1.5 DM. Thiscopper powder can be produced, for example, by precipitating Cu metal inan aqueous medium by reduction of Cu ions in the presence of Sn ions. Itis preferably produced by reducing cuprous oxide of an Sn content of10-1,000 ppm in an aqueous medium. Note that as termed in thisspecification, “particle diameter” means the particle diameter measuredalong the major axis of the particle.

This invention enables improvement of the weatherability of a copperpowder by means of incorporating Sn therein. The copper powder can beefficiently produced by a method capable of utilizing electrolyticcuprous oxide as the Cu sarting material developed by the inventors,using Sn contained in the electrolytic cuprous oxide as impurity,without need for any special process for weatherability improvement. Byoptimizing the Sn content, a copper powder having the propertiesfundamentally required by a copper powder for conductive paste can beobtained. The present invention therefore contributes to improvement ofelectronic equipment reliability by providing a copper powder forconductive paste imparted with weatherability and high in costperformance.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph showing an example of theappearance of the copper powder of the present invention.

FIG. 2 is a graph showing the results of a weatherability test.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The inventors learned through various studies that a copper powdercontaining Sn can be obtained by reducing Cu ions in the presence of Snion in an aqueous medium. It was further discovered that a copper powdercontaining an optimal amount of Sn exhibits a property of stronglyresisting bonding with oxygen such as during storage in air at roomtemperature, i.e., exhibits excellent weatherability by which change inthe surface state owing to progressive oxidation does not readily occur.A copper powder excellent in such weatherability can be produced usingthe copper powder production process developed by the inventorsexplained in the following.

Use of electrolytic cuprous oxide as the Cu starting material isadvantageous from the viewpoint of easy availability and low cost.However, when cuprous oxide is reduced by a conventional productionprocess, the particle size distribution of the obtained copper powderdepends strongly on the particle size distribution of the cuprous oxide.That is, the reaction rate is affected by change in the surface area ofthe cuprous oxide with variation in its particle diameter so that theparticle diameter of the obtained copper particles also varies. This isparticularly true when using coarse cuprous oxide of a particle diameterof several μm or greater, because the small specific surface area of thecuprous oxide makes the reaction rate slow for the scale of the Cusource mass, so that the particle diameter of the copper particles alsotends to become large. Electrolytic cuprous oxides generally availableon the market are of irregular particle shape and vary in particle sizedistribution. Copper powder of constant particle diameter is thereforevery difficult to produce with good reproducibility by a conventionalreduction method using electrolytic cuprous oxide as starting material.

Based on the results of in-depth research, the inventors developed amethod comprising the steps of preparing a mixed liquor obtained bymixing electrolytic cuprous oxide as the principal starting material anda more readily soluble water-soluble copper salt in an aqueous medium,causing a reducing agent to act on the mixed liquor to inducepreferential early precipitation of Cu derived from the copper salt, andusing this Cu as nuclei for precipitating Cu derived from theelectrolytic cuprous oxide. This method makes it possible to producefine copper powder of controlled particle size unaffected by theparticle size distribution of the electrolytic cuprous oxide.

Thus in this method, before reduction of the cuprous oxide by thereducing agent, the Cu ions liquated out of the water-soluble coppersalt that reacts more readily than the cuprous oxide rapidly react withthe reducing agent to form a nuclei for particle growth. Next, Cu ionsliquated out of the particle surfaces of the cuprous oxide that is theprincipal starting material are reduced and precipitate onto the nuclei.At this time, the reduction reaction of the cuprous oxide precedes quitegradually so that spherical copper particles of uniform particle sizeare formed. Therefore, the particle diameter of the obtained copperparticles is determined by the number of the nuclei for growth and doesnot depend on the particle size distribution of the cuprous oxide. Inother words, the average particle diameter of the obtained copper powderis determined by the mass of the starting material cuprous oxide and thenumber of the nuclei and the range of the particle size distributionthereof is narrow. Minute observation showed that the precipitateconstituting the nuclei for growth consisted of aggregated copperparticles of a particle diameter of 20-50 nm.

It is important here to add a protective colloid to the liquor inadvance, before allowing the preferential reduction reaction of thewater-soluble copper salt to occur. The size of the aggregates can becontrolled by varying the amounts of copper salt and protective colloidadded. Specifically, a large number of small sized aggregates areproduced when the amounts of copper salt and protective colloid arelarge, so that the particle diameter of the finally obtained copperparticles becomes small. To the contrary, when the amounts of coppersalt and protective colloid added are small, a small number of largesized aggregates are produced, so that the particle diameter of thefinal copper particles becomes large. This principle can be used tocontrol the particle diameter of the copper particles, thereby enablingproduction of a fine copper powder of uniform particle diameter evenwhen using a cheap electrolytic cuprous oxide of irregular particleshape and size as the starting material.

The procedure followed can be either to mix the cuprous oxide,water-soluble copper salt and protective colloid by stirring in anaqueous liquor and add the reducing agent to the mixed liquor or to mixonly the water-soluble copper salt and protective colloid together, addthe reducing agent to the aqueous liquor obtained to produce the nucleiin advance, and then add and reduce the cuprous oxide that is theprincipal starting material for Cu particles.

Electrolytic cuprous oxide generally available on the market contains Snas an impurity. When the aforesaid reduction and precipitation onto thenuclei occurs, Sn liquates out of the starting material electrolyticcuprous oxide together with Cu. This means that Cu ions are reduced inthe presence of Sn ions to precipitate as copper metal. It is reasonableto conclude that at the time of Cu metal precipitation the Sn componentof the liquor is taken into the interior and onto the surface of thecopper particles. The mechanism by which the presence of Sn in thecopper particles improves the weatherability of the copper powder is notclear on number of points but it is thought that the Sn forms adistinctive oxide coating on the copper particle surfaces and thiscoating exerts an effect of inhibiting oxidation of the copper.

Experimentation showed that the effect of improving the weatherabilityof the copper powder produced by the Sn content is pronounced at an Sncontent exceeding about 10 ppm. A marked weatherability improving effectemerges in the Sn content range of 10-100 ppm and becomes extremely highup to at least 2,000 ppm. Moreover, a weatherability improving effectcan be enjoyed up to around 20,000 ppm (2 mass %). However, caution isnecessary when the Sn content exceeds 20,000 ppm because the resultingdecline in the purity of the copper powder is liable to have an adverseeffect on the electrical and other properties of the copper powder. TheSn content of the copper powder is affected by the amount of Sncontained in the electrolytic cuprous oxide that is the principalstarting material. When the amount of Sn therein is insufficient, itsuffices to add a stannic salt. This makes it possible to control the Sncontent of the copper powder to an appropriate level.

The cuprous oxide used as the principal starting material is preferablyan electrolytic cuprous oxide containing Sn at about 10-1,000 ppm. Useof electrolytic cuprous oxide of an average particle diameter of around3-10 mμ is particularly preferable from the viewpoint of productioncost.

Although the copper salt used as the secondary starting material isrequired to be water soluble, it can be any of various types. Among suchcopper salts, monovalent copper salts are preferable because they makethe precipitation of nuclei more uniform. Typical examples of thesepreferable monovalent copper salts include cuprous acetate, cuprousnitride and cuprous chloride. The amount added is preferably about0.1-20 mole % per 100 mole % of the principal starting material cuprousoxide. When the amount added falls below 0.1 mole %, the effect ofimpurities in the starting material becomes large to lower productionstability. On the other hand, addition of more than 20 mole % isuneconomical because the particle diameter of the copper powder does notchange substantially no matter how much copper salt is added in excessof this level.

The protective colloid used can be selected from among such commonwater-soluble polymers as gum arabic, polyvinyl alcohol, polyethyleneglycol, polyvinylpyrrolidone, gelatin and the like. The amount added ispreferably about 0.1-1.0 parts by mass per 100 parts by mass of thecuprous oxide. This enables the average particle diameter DM of thecopper particles to be controlled to within the range of 0.1-2 μm or,further, the range of 0.2-1 μm.

Usable reducing agents include hydrazine, hydrazine hydrate, hydrazinecompound, formaldehyde, sodium borohydride and the like. Hydrazine andhydrazine hydrate are preferable in the points of reducing power andhandling ease. The amount added must be enough to completely reduce thestarting materials but is preferably about 50-300 mole % relative to thetotal amount of Cu. Addition in an amount below this range causes thereduction reaction to proceed too slowly, and addition in an amountabove this range causes the reaction to become so vigorous as to makeparticle diameter control difficult and is also uneconomical. Additionat the rate of 80-150 mole % relative to the total amount of Cu isparticularly preferable.

During the reduction reaction, particularly at the particle growthstage, a complexing agent is preferably added in order to stablygenerate and supply Cu ions. Tartaric acid, acetic acid, citric acid andammonia and their salts, for example, can be used as the complexingagent and added to the reaction liquor as appropriate. The Sn content ofthe copper powder can be controlled by adding a stannic compound such asstannic oxide, stannic chloride or the like.

The temperature during reduction is preferably maintained at around30-80° C. The reduction reaction proceeds too slowly at below 30° C. andat above 80° C. it becomes too vigorous, which promotes generation ofsecondary nuclei and makes control of particle diameter difficult. Atemperature in the range of 40-60° C. is still more preferable.

It is generally considered that a copper powder for conductive pasteshould consist of fine (small diameter) particles and have a narrowparticle size distribution. The average particle diameter DM ispreferably 0.1-2 μm, more preferably 0.2-1 μm. On top of meeting the DMrequirement, it is preferable for the particle diameter of at least 80%of all particles of the copper powder to fall in the range of 0.5 DM-1.5DM, more preferably for the particle diameter of at least 80% of allparticles to fall in the range of 0.7 DM-1.3 DM. The particle sizedistribution can be so regulated by using the production methodexplained in the foregoing.

EXAMPLES Example 1

Electrolytic cuprous oxide of an average particle diameter of 3 μm wasprepared. The prepared electrolytic cuprous oxide had a broad particlesize distribution, i.e., 50% or more of all particles fell outside therange of 3 μm±1 μm. The Sn content of the electrolytic cuprous oxide was0.01 mass %. This electrolytic cuprous oxide, 135 g, was dispersed in3,750 g of pure water. The dispersion was added with 7.5 g of cuprouschloride as water-soluble copper salt and 15 g of polyvinyl alcohol asprotective colloid and then heated to 40° C. under stirring. To theheated mixture were added 100 g of 80% hydrazine hydrate as reducingagent and 22.5 g of acetic acid as complexing agent. The resultingliquor was heated to 60° C. over one hour and then held at 60° C. foranother hour to allow the reduction reaction to proceed. The liquorafter reaction was subjected to solid-liquid separation and therecovered solids were washed with water and dried to obtain a copperpowder. The copper powder was observed under a scanning electronmicroscope (SEM) and the diameters of the particles within the field ofvision were measured. It was found that the average particle diameter DMwas 0.3 μm and that the particle diameter of at least 80% of allparticles of the copper powder fell in the range of 0.5 DM-1.5 DM. Ascanning electron micrograph of the copper powder is shown in FIG. 1.

The copper powder was dissolved in acid and subjected to compositionalanalysis by ICP spectrometry. The Sn content of the copper powder wasfound to be 120 ppm.

Example 2

To 3,750 g of pure water were added 7.5 g of cuprous chloride aswater-soluble copper salt and 15 g of polyvinyl alcohol as protectivecolloid. The result was heated to 40° C. under stirring, whereafter 100g of hydrazine hydrate was added as reducing agent. To the resultingreaction liquor was added 135 g of the same electrolytic cuprous oxideas used in Example 1, 0.43 g of stannic chloride as stannic salt and22.5 g of acetic acid as complexing agent. The resulting liquor washeated to 60° C. over one hour and then held at 60° C. for another hourto allow the reduction reaction to proceed. The liquor after reactionwas subjected to solid-liquid separation and the recovered solids werewashed with water and dried to obtain a copper powder. The copper powderwas observed under a scanning electron microscope (SEM) and thediameters of the particles within the field of vision were measured. Itwas found that the average particle diameter DM was 0.3 μm and that theparticle diameter of at least 80% of all particles of the copper powderfell in the range of 0.5 DM-1.5 DM.

The copper powder was subjected to the same compositional analysis asthe copper powder of Example 1. The Sn content of the copper powder wasfound to be 1,900 ppm.

Comparative Example 1

Copper sulfate, 110 g, was dissolved in 330 g of pure water, thesolution was neutralized by adding 90 g of sodium hydroxide, followed byaddition of 440 g of 60% glucose solution. Cuprous oxide wasprecipitated by a reduction reaction progressing at 70° C. Hydrazinehydrate, 120 g, was added to the resulting cuprous oxide slurry and theslurry was heated to 90° C. over 3 hours to allow a reduction reactionto proceed. The liquor after reaction was subjected to solid-liquidseparation and the recovered solids were washed with water and dried toobtain a copper powder. The copper powder was observed under a scanningelectron microscope (SEM) and the diameters of the particles within thefield of vision were measured. It was found that the average particlediameter DM was 0.3 μm.

The copper powder was subjected to the same compositional analysis asthe copper powder of Example 1. The Sn content of the copper powder wasfound to be 3 ppm.

Weatherability Test

The copper powders obtained in Examples 1 and 2 and Comparative Example1 were individually exposed to atmospheric air in a thermostaticchamber. After a fixed time period, their oxygen amounts were measuredby the method of fusion in an inert gas and infrared ray absorption,thereby ascertaining the time-course change in oxygen absorption amountin a 25° C., R.H. 30% atmosphere. The results are shown in FIG. 2.

As can be seen in FIG. 2, the amount of oxygen absorbed at roomtemperature by the Sn-containing copper powders of the Examples was verylow, so that they exhibited outstanding weatherability. In contrast, thecopper powder of the Comparative Example, which contained almost no Sn,absorbed an increasing amount of oxygen over the course of time and wasthus inferior in weatherability.

1. A copper powder containing 10-20,000 ppm of Sn.
 2. A copper powdercontaining 100-2,000 ppm of Sn.
 3. A copper powder according to claim 1,whose average particle diameter DM is 0.1-2 μm.
 4. A copper powderaccording to claim 1, whose average particle diameter DM is 0.1-2 μm andwherein the particle diameter of at least 80% of all particles is in therange of 0.5 DM-1.5 DM.
 5. A copper powder according to claim 1,obtained by precipitating copper metal by reduction of Cu ions in thepresence of Sn ions.
 6. A copper powder according to claim 1, obtainedby precipitating copper metal by reduction of cuprous oxide containing10-1,000 ppm of Sn.
 7. A copper powder suitable for use as filler in aconductive paste, containing 10-20,000 ppm of Sn, whose average particlediameter DM is 0.1-2 μm and the particle diameter of at least 80% of allparticles is in the range of 0.5 DM-1.5 DM.
 8. A copper powder accordingto claim 2, whose average particle diameter DM is 0.1-2 μm.
 9. A copperpowder according to claim 2, whose average particle diameter DM is 0.1-2μm and wherein the particle diameter of at least 80% of all particles isin the range of 0.5 DM-1.5 DM.
 10. A copper powder according to claim 3,whose average particle diameter DM is 0.1-2 μm and wherein the particlediameter of at least 80% of all particles is in the range of 0.5 DM-1.5DM.
 11. A copper powder according to claim 2, obtained by precipitatingcopper metal by reduction of Cu ions in the presence of Sn ions.
 12. Acopper powder according to claim 3, obtained by precipitating coppermetal by reduction of Cu ions in the presence of Sn ions.
 13. A copperpowder according to claim 4, obtained by precipitating copper metal byreduction of Cu ions in the presence of Sn ions.
 14. A copper powderaccording to claim 2, obtained by precipitating copper metal byreduction of cuprous oxide containing 10-1,000 ppm of Sn.
 15. A copperpowder according to claim 3, obtained by precipitating copper metal byreduction of cuprous oxide containing 10-1,000 ppm of Sn.
 16. A copperpowder according to claim 4, obtained by precipitating copper metal byreduction of cuprous oxide containing 10-1,000 ppm of Sn.